Activation of protein kinase C (PKC) can modulate intracellular cAMP levels. Whether this takes the form of an enhancement or an attenuation of cAMP generating responses is determined by the specific PKC isozyme(s) expressed in cells. We investigated the role of PDEs as mediators of this modulation in NIH 3T3 cells where the expression of specific PKC isozymes was manipulated by 1) transfection and 2) activation of PKC by the phorbol ester PMA. Our results suggest complex interactions between PKC expression and PDE regulation: a) Transfection effect: In control cells lacking gamma PKC, total PDE activity is relatively low and cGI PDE is absent. Upon transfection with gamma PKC, total PDE increases dramatically and cGI PDE appears, suggesting the regulation of the PDE gene expression by gamma PKC. b) PMA effect: In the control cells, activation of PKC (by PMA) has little effect on PDE activity (and therefore cAMP modulation). However, in the transfected cells activation of PKC results in a decline of PDE activity. This suggests possible acute regulation by phosphorylation of PDEs by PKCgamma since PMA has little effect in control cells or cells transfected with PKCbeta. Whether PMA (PKCgamma) alters all or specific PDEs in the transfected cells is under investigation.